A switchgear is basically a circuit breaker or a general purpose switch of a larger scale, having the function of making it possible to cut off the current to a certain cable in large power networks and in industries. Such a switchgear comprises a circuit breaker and typically also a disconnecting switch or a general purpose switch. The entire switchgear is usually connected to ground, as a security measure.
Switchgears are used in electric power systems, with the purpose to control, protect and isolate electric equipment. In distribution nets, switchgears are located both on the high voltage side and the low voltage side of power transformers. The switchgear on the low voltage side of the transformers may be located in a building, with medium voltages breakers for distribution to end users by means of several individual circuits.
As mentioned, one of the basic functions of switchgears is protection, which includes interruption of short-circuits and overload fault currents while maintaining service to other unaffected distribution circuits in the net. An arc suppressor/eliminator is often provided in the switchgear in order to decrease or, preferably, eliminate any damages that may be caused by faults. An arc suppressor may short out an electric arc within a switchgear, and in doing so it may also prompt a breaker, located up-stream in the net, to clear the fault by cutting the power to the switchgear. Such an arc suppressor may comprise a knife switch, having a pivoting contact knife that is movable in order to connect the current carrying conductor to ground. Such an arc suppressor may also be called an earthing switch.
Switchgears used today in electric power distribution systems for medium and/or high voltages, e.g. 1-1000 kV, such as 12, 24 or 36 kV, are usually gas insulated switchgears (GIS) where the used insulating fluid is Sulphur hexafluoride (SF6). The switchgear is located in a sealed encapsulation, filled with the insulating fluid. In case of an internal arc in the switchgear, hot gasses resulting from vaporization of conductors, insulation and enclosure will flow from the arc area to the surroundings. These gases will increase the pressure inside the encapsulation that will eventually result in a blowout. In order to prevent the buildup of too much pressure, an arc suppressor can be introduced to minimize the arcing time. This arc suppressor may be of the knife switch type. The challenge in obtaining a successful use of arc suppressors is to ensure fast and adequate contact between the arc suppressor, i.e. the contact knives, and the conductors carrying the arc fault current. If the contact is not fast and adequate enough, this may cause a flash-over.
Today the solution is to add a non-conductive guide member, e.g. of plastic, on the current carrying conductor, where the contact knives are intended to come into contact with the conductor. The function of this guide member is to guide the knives to ensure fast and adequate contact (large enough contact area) between the knives and the conductor. The knives must make contact with the conductor at the right place, not too early and not too late, in order to have large enough contact area. However, this arrangement has the disadvantage that triple points may occur where the guide member is located on the current carrying conductor, due to a potentially very high electric field strength where a non-conductive material such as plastic is close to metal. This may result in starting partial discharges or electric flashovers. Another disadvantage is that the guide member absorbs some of the kinetic energy from the contact knives when the moving contact knives hit the guide member, and then the contact knives are pressed apart before they can pinch onto the conductor. This may result in too early pinching of the contact knives onto the conductor. The contact knives will weld immediately to the conductor resulting in a too small contact area.